Factors Affecting Stall Speed

[niallcooney]

Another point. NorthernSky, I think u misinterpreted something about autopilots here. If I'm not mistaken John wasn't talking about autopilots avoiding the stall, he was talking about the autopilots fly the aircraft. Autopilots trim for alpha, and that's a fact. Trimming for speed in a heavy airliner with a constantly changing CoG and possible Mach tuck due to center of pressure shifts above 0.615M (in a 737) would present serious problem as weight shifted etc. Trimming for alpha means that the autopilot is 'thinking ahead' of the aircraft, relying on actual performance rather than what's indicated on the speed tape which as John mentioned can have built in or pitot and static position errors leading to an incorrect reading, whereas an alpha reading would negate the need for an airspeed indicator at all! Most V speeds are alpha related... Vs0, Vs1, Vno, Vx, Vy, the list goes on. But it's safe to say that 'stalling speed' is a VERY dangerous choice of words to teach a student. Get a student to pull 3G's at 90 knots in a Cessna 150 and ask him to explain the flick roll and spin that follows if he's been taught about 'stalling speed'!

[Checkboard]

A bit pedantic, this discussion, I think. It's pretty obvious that we are talking about changes in the observed IAS during a stall, with Vs as a reference.. .. .Some factors not yet mentioned:. .. .</font><ol type="1">[*]<font size="2" face="Verdana, Arial, Helvetica">Ground effect</font></li>[*]<font size="2" face="Verdana, Arial, Helvetica">Formation flight</font></li>[*]<font size="2" face="Verdana, Arial, Helvetica">Damage to the wing</font></li>[*]<font size="2" face="Verdana, Arial, Helvetica">Compressibility</font></li>[*]<font size="2" face="Verdana, Arial, Helvetica">Climb/Descent angle</font></li>[*]<font size="2" face="Verdana, Arial, Helvetica">Boundary layer control devices ("blown" wings and vortex generators)</font></li>[*]<font size="2" face="Verdana, Arial, Helvetica">Vectored thrust</font></li>[*]<font size="2" face="Verdana, Arial, Helvetica">Gusts</font></li>[/list=a]<font size="2" face="Verdana, Arial, Helvetica">

[vmommo]

Put simply stall speed is determined only by wing loading and the maximum lift coefficient. . .. .However, there are multiple ways to vary those two values, most of which have been discussed above.

[vmommo]

SentryIP, back to you intial question. Published AFM stall speeds are for forward CG, which are worst case senario, in other words highest value (fastest). . .. .Also, add air density to my above post.

[TowerDog]

Stalling ALPHA?. .. .We are not talking Airbus logic here.. .. .Hey, slow down guys.. .I was asking what time it is and some of ya studs are building the clock.. .. .Checkerboard:. .. .Yer Number 4 was indeed mentioned above... .. .As for ground effect, yup, that would do it.. .. .Formation flight and damage to the wing?. .Well. Both would change the airflow and so on.... .So would a mid-air collision or a drunk pilot hanging out of the side window and on and on... .Uh slow down here guys.... .. . <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />

[Genghis the Engineer]

I don't think anybody's actually answered the question, which was is Vs affected by CG. The answer is yes.. .. .The aerodynamic stall, where flown breaks down completely over the wings is rarely reached by any aeroplane. This is because the "piloting" stall occurrs when, whilst still nibbling the aerodynamic stall, you run out of nose-up control authority. This is a function of CG.. .. .So, with further forward CG, you have less ability to pitch up, less ability to enter the aerodynamically stalled regime - and a higher perceived stalling speed. This pitch limited stall is referred to by a lot of pilots as "mush".. .. .G

[NorthernSky]

ATPLwannabeboy,. .. .I fly the 737 at present, and I assure you that the autopilot does not trim for mach tuck. This is achieved by the Mach Trim system which then commands re-datuming of the Elevator Feel and Centering Unit to move the elevators. As it happens the system is armed above 0.615M, but the aircraft can be flown to (3-4-5) 0.74M (NG) 280kts/0.82M with the system inoperative, without trouble.. .. .I can assure you that the 737 autopilot system does not use AoA information.

[niallcooney]

Right, anyone else agree that the 737 autopilot DOESN'T trim for alpha? (Anyway CoP change causes an alpha reduction resulting in a nose-down moment) If it doesn't it's one strange and clunky system. However if no-one else contributes we'll just have to take Northern's word for it. Any 737 drivers care to comment? TowerDog, stalling alpha (or if u want to call it AoA) is what the 'stalling speed' is measured on, whether it's a 73clunk or an A320... we all know that <img border="0" title="" alt="[Wink]" src="wink.gif" />

[Hotel Charlie]

ATPLwannabeboy!. .. .I´m afraid I have to agree with Northernsky. . .As he I also fly the 737 (4/5/700) and unless they (Boeing) have left it out of the books he is right!

[niallcooney]

Thanks for clearing that up, HC and grudging apologies to Northern. Any other airliners that don't trim for alpha (and I'm not talking about what it APPEARS to be trimming for)? I'm still convinced that MOST autopilots trim for alpha, and that alpha is what we should be talking about in terms of a stall.

[NorthernSky]

ATPLwannabeboy,. .. .In answer to your latest question, I will go so far as to say that airliner FCSs do not reference AoA, with the exception of some oddities, and modern 'busses - A320, 330, 340 families, which use it for envelope protection. The oddities I mention would, I think, include the L1011 - though never having flown one, I'm going on what people have said about some of its modes, which I guess must have been referenced to AoA (DLC, perhaps?). Any L1011 drivers care to comment?. .. .Or to put it more simply, you may quote me on this: 'In normal flight, airliner autopilots pay no heed to AoA'.. .. .BTW, I'm not having a go at you, wannabe, or anyone else. It's important that people who fly these aircraft know what their machines are doing, and why. Too often, lack of this understanding leads to mishap, especially with regard to AFCS. I'm just trying to get the true facts across, as part of this lively and enjoyable debate.. . . . <small>[ 04 March 2002, 18:03: Message edited by: NorthernSky ]</small>

[John Farley]

NorthernSky. .. .Thanks for putting everybody right about my duff gen. Good job.. .. .What I should have said was "All auotpilots and Flight Control Systems (that include a stall protection capability in their spec) use AoA to avoid the stall" . .. .Sometimes in an attempt at brevity it is all too easy to finish up misleading others, which is an extremely undesirable thing in aviation.. .. .Regards

[ColdnFoggy]

An interesting thing about CG location is the difference in stall characteristic between an aft and a forward CG.. .When CG is forward the stall is very "honest" eg. plenty of pre-stall warning( Buffet). With an aft CG my expierence is that the thing barely gives you any buffet and the stall is more violent.. .. .And now to the Techies: How does this affect the way a modern "computerized" airplane presents it protections vs stall? Is the margin to stall the same in both cases, or could you expect to experience buffet at max AOA on a forward CG aircraft.( eg. Valfa max on an Airbus)?

[BOAC]

Excuse thread creep but calling JOHN FARLEY - I cannot email you via the icon - can you email me with an email address please? Breathe is 'bouncing'.. .Mike Phillips

[TheAerosCo]

NorthernSky has explained why the stall speed, if we must call it that, is not zero in the vertical. However, to be pedantic I can think of a couple of different situations where it could be said that the stall (i.e exceedance of the critical angle of attack)can occur at zero indicated airspeed. Any thoughts? To give a clue, I expect John Farley will get one of them pretty quick.

[Justappl]

8?. .. .Angle of Bank

[ft]

TheAerosCo,. .I'm curious as to how you would you define angle of attack at zero airspeed?. .. .Cheers,. . /ft

[Lima Xray]

NorthernSky . .. .“If it did approach the stall, or if you did the same thing in a turboprop, for example, the Autopilot would be thrown out by the stick-shaker or stick-pusher”.. .. .Not everyone fly's the fancy stuff.... .In a bog standard turboprop with standard Autopilot experiencing icing conditions. Operating in ALT mode the A/P whilst trying to keep altitude slows the A/C down. With the stall warner being ices up by that time. What will happen? It’s a killer situation, which has cost lives. No automatic devices around....other then a shaken pilot.. .. .Love me feed me

[NorthernSky]

John Farley,. .. .Thanks for your kind remarks. This is a great way to debate and discuss, isn't it?!. .. .Lima, you make a valid point (and one which I understand, especially having flown the ATR - I thought it was a wonderful aircraft, btw). However, you have to realise that (a) the authorities set a standard, then, (b) a manufacturer builds an aircraft which meets the standard, then (c) people go flying in it. Multiple failures are not accounted for in these standards, and it is 'assumed' that systems will usually work as advertised.. .. .OK, bad things have happened in turboprops, but this does not mean that every turboprop which flies into icing will suffer the fate which you propose. I have some great photographs of the 'Ice detector' on the ATR, covered with sparkly white stuff (about two inches of it), to prove my mettle and my belief in the systems which were protecting me at the time. However, I was also 'ice aware', and would not have climbed the aircraft in V/S in those circumstances. Knowledge may not be power, but it certainly helps awareness.

[Pegasus77]

I have seen angle of bank mentioned twice now as influencing the stall speed: this is not true. The higher g-load in a turn causes the stall speed to be higher, not the bank angle. . .. .Also mentioned was climb or descent angle. These are not influencing the stall speed. It is the AoA, where it really doesn't matter if you are climbing or descending.. .. .P77